This invention relates to signal processing apparatus, and more particularly to interfacing apparatus for analyzing and displaying the output signals from a gas chromatograph.
The present invention is applicable generally to the processing and display of electrical signals, for example where it is desirable to analyze and display the waveform of a signal with a wide dynamic range and minimize the effect of any noise on the signal. It is particularly well suited to the analysis of the hydrocarbon content of drilling mud as it is circulated into and out of an oil or gas well during rotary drilling operations. During the rotary drilling of a well, the drilling mud, which is a suspension of oil, water, and various solids, is pumped down the hollow drill string to the bottom of the well, exits through holes in the drill bit, and returns to the surface through the annular space between the drill string and the borehole wall. Among the functions of drilling mud are cooling the drill bit, carrying rock cuttings to the surface, and sealing the borehole wall. It has also been recognized that the drilling mud returning to the top of the well often contains detectable quantities of any hydrocarbons that are present in the pore spaces of the underground formations penetrated by the borehole. Analysis of the mud is therefore useful to determine the hydrocarbon content of subterranean strata currently under exploration.
The detection and differentiation of the various hydrocarbon components present in drilling mud is most rapidly and economically accomplished by gas chromatography. A sample of a gaseous mixture to be analyzed is introduced into a column containing a medium which retards the passage of the individual components of the sample to differing degrees that are roughly correlated to the logarithms of the respective molecular weights of the components. A carrier gas is directed through the column to elute the constituents from the medium in sequence. At the output end, the quantity of each component is then measured by a gas detector, for example of the flame ionization type, such as that used in the Varian 940 gas chromatograph. Alternative types such as the hot-wire detector, which measures the thermal conductivity of the effluent gas from the column, may also be used. The column is calibrated by passing known, pure gases through it and measuring the peak amplitudes of the respective hydrocarbon components.
The operating cycle of a chromatograph generally comprises at least a forward phase, in which the flow of carrier gas is maintained in the forward direction long enough for all detectable components of the gas mixture to be eluted. There is also a reverse phase in which the column is backflushed in order to ready the column for the next sample. As an alternative, if it is desired to measure only lighter components, the flow can be reversed after the desired hydrocarbons have been eluted and measured, and only the heavier residual components are backflushed into the atmosphere. In yet another system, the heavier ends are conducted after backflushing not to the atmosphere, but rather to the gas detector for analysis in bulk.
The thermal conductivity cell of a hot-wire detector or the electrometer of a flame ionization detector produces as the output of the chromatograph an analog signal whose voltage is representative of the quantity of materials detected at a particular point in time. This signal is generally displayed on a chart recorder in the form of a chromatogram, which is a graph including one or a plurality of curves, each peak to the curves corresponding to one detected component of the specimen. The composition of the specimen can then be determined by comparing the chromatogram of the specimen with previously produced chromatograms of a known substance, because the location of each peak on the chromatogram indicates the elution time between the commencement of measurement and the arrival of the component substance at the output end of the chromatograph tube, and thus indicates the unique molecular weight of the component substance. Quantity and concentration data can be determined from the amplitude of the chromatogram curve over time.
In order to measure component quantities which may vary by many orders of magnitude, attenuating or amplifying devices are typically associated either with the chromatograph, the chart recorder, or an interface or signal processing device linking the two. This feature brings the output signal to a level within the full-scale range of the recorder and is necessary in order to keep the pen on the chart and to have a readable curve for both very large and very small concentrations of a component. These scaling devices may be either manual or automatic, and include either manual switches or analog or digital devices that perform the functions of sensing the range of the magnitude of the output signal and attenuating or boosting the signal accordingly.
Analog or digital systems are also used to electronically integrate the signal over time for each peak, thus determining the area below the chromatogram curve, and above a baseline level, the latter level representing the output signal owing solely to the detected carrier gas, plus noise. This area is a measure of the quantity of component substance detected, and thus can be used to determine the concentration of each component substance in the total specimen. Autozero devices may be used to set the baseline curve at the zero point on the output graph.
This invention is intended to address certain recurring problems with prior chromatograph interfaces. First is the problem of noise. As stated previously, the desired analysis of a test specimen can be derived from the sequential series of peaks of a chromatogram on the basis of, first, the peak emergence time to identify the components of the sample, and second, the amplitude of the peak to indicate their concentrations. Peaks may be wall resolved, or, on the other hand, a chromatograph may be complex, containing many peaks, some only partially resolved. The digital or analog device that processes the signal must sense the occurrence of a peak, measure the peak amplitude, provide for separating overlapping peaks, and allow for the baseline signal.
Each of these functions can be impaired by noise. One type is the ordinary 60 Hertz line noise from power supplies, amplifiers, and readouts. Electrical noise can also arise from voltage surges from sources unrelated to the chromatograph, or internal spikes, for example from particulate matter entering a detector. High-frequency detector noise may also be produced, for example, by impurities or flow irregularities in the fuel gas of a flame ionization detector, or by sensitivity to carrier gas flow or filament movement in a hot-wire detector.
Pneumatic noise may arise from an unwanted baseline change due to loss of retarding medium, or changes in the flow of the carrier gas either inherent in the pumping or regulating devices or due to leaks or faulty components. Chemical or physical noise can be due to contamination of the chromatograph column or displacement of the materials therein.
Prior systems have required that complex smoothing, weighting, or slope-averaging functions be performed in order to determine whether spikes or flat signal portions are noise or signal. It is desired that an apparatus be provided to compensate for noise in a simpler way.
Another problem to be addressed is flexibility of display and analysis of the chromatograph signal. In conventional display systems the output to the chart recorder commonly is switched either automatically or manually to a range appropriate to the magnitude of the signal. It would be desirable for the number of such ranges to be minimized, for the dynamic range of signals that can be accommodated without such switching to be increased, and for the operator to be informed when the measurement range of the instrumentation has been exceeded.
Further, it is desirable that the signal analysis device be easily adaptable to changes in the mode of analysis, have automatic features enabling it to operate automatically to the greatest extent possible, and be accurate and reliable.